Nonny, you have awoken the beast called Obsessions of my Youth and Iove you for it.
I’ve studied a variety of fighting methods, through martial arts to the sport of fencing, and sword fighting was the one I spent literally hours reading about as a small child. I have already put this to use in a similar ask on writing fight scenes in general, all of which is applicable to writing a sword fight in the same way it is to writing a fist fight, brawl etc. That being said, it only half answers your question; the other half is the small technicalities of the weapon itself, and how this impacts your writing.
Writing really good sword fighting scenes doesn’t require an in-depth knowledge of swords, per se, but having that knowledge can be extremely helpful. There are different makes of sword and each one lends itself to a different style of fighting; and the way you write the scene can be influenced by that style.
In the Song of Ice and Fire books, the ways that Jon and Arya fight are very different, although they both “sword fight”. Arya has a very long, thin needle-tipped blade which lends itself to a style that is dancelike and exact, and very precise in its brutality. There would be no slashing if you were to write a sword fight with a sword like that one. The model is that of a foil, the only point of which is the…well, the point. “Stick them with the pointy end” is the joke about this sword in the books, and in the real life counterpart that is exactly what you would do. So, in this case, you might write a sword fight like this:
Back, back, from toe to heel and trying not to slip in the mud. She parried and his blade slid along hers, shaving the length near the hilt and allowing her underneath his arm. A flash of leather, joining cloth. Vulnerability. She pushed in, her body propelling the point forwards, and wound herself back as soon as blood had been drawn.
It would be faster, choppier, mimicking the rhythm that a fighter with this weapon would have to adopt in order to be successful. Words like “slashed” or “hacked” wouldn’t be used either. Actually, slashing or hacking are kind of redundant anyway, whatever the sword, because the art of sword fighting is a lot more refined than that.
On the other hand, if your character were to fight with a larger sword such as the typical English medieval broad sword, the whole thing would be written in longer sentences to tie in with the feel of a heavier blade for both stabbing and cutting.
One last point is to remember how hard it is to kill a person. So often I read works, both published and unpublished, original and fandom, where the swords “slice clean through” or are “thrust into his heart” or, my personal favourite, “slide through bone and gristle.” (this last usually in the context of severing a limb, or the head.).
Aside from steel armour, the body does have some armour of its own. In the Last Kingdom books, this is something that is adressed so well; I’ll never forget the way that Cornwell wrote Uhtred realizing that killing someone was harder than it looked. It takes an awful lot of physical strength to weild the traditional sword alone, never mind plunging it through someone’s breastbone or cutting off their head. Not to mention, if your sword fight is taking place in a battle with other trained soldiers or warriors, the character would be up against getting their sword through even more muscle and sinew than that of the average person. Sword fighting is not graceful and easy, no matter how proficient someone is at it, and so it shouldn’t be written that way. It is a hard, highly skilled, physically demanding and overall incredibly brutal. It’s difficult to maintain finesse when you’re fighting for your life with lots of other sharp, pointy things being swung around you and at you, and you need to remember every technique you have been taught in order to injure your opponent just enough to put them out of action. Even in duels, death was uncommon. It was more likely to be terrible injury, because in the rush of action there simply isn’t the time to only aim for killing blows.
Hope that this can be of help, and best of luck with your writing
ʕ •ᴥ•ʔゝ☆